The present invention relates to the communication of digital video signals, and more particularly, to the insertion of digital video messages such as commercials into a pre-existing compressed packetized data stream. Data packets of the commercial message are spliced into a pre-existing data stream (complying, e.g., with the Moving Picture Experts Group (MPEG) transmission standard) without decompressing the data in the data stream, and while maintaining compliance with the MPEG or similar digital data communication protocol.
Digital transmission schemes are particularly advantageous for signals that are broadcast from a main office by satellite to a cable television affiliate at a system headend. At the system headend, the digital data stream may be further processed and distributed to the cable system customers, for example, via a Hybrid Fiber Coax (HFC) or Fiber to the Curb (FTTC) network. Such an arrangement is known as an end-to-end digital network since digital video is compressed and transmitted from a programming source at a central office all the way to each customer's home. In an HFC network, a distribution line includes both a coaxial cable which carries radio-frequency signals, and an optical fiber which carries light wave signals. In a FTTC network, an optical fiber carries the data stream from the cable system headend to a neighborhood transfer point, or drop, and conventional coaxial cable carries the signal from the drop to the customer's home. At the customer's home, a decoder processes the digital signal to provide a signal for display on a television or other display device.
In such cable distribution systems, the received data stream may be processed at the headend prior to distribution to the system customers. In particular, commercial messages from local businesses may be inserted into the main programs. However, in order to accomplish this, conventionally the received digital data stream must be completely demodulated, demultiplexed, decrypted, decompressed and decoded to recover the signal in the analog domain. Then, the desired commercial message is provided in the analog domain and inserted into the signal to provide a new analog combined signal. Finally, the analog combined signal is digitized, encoded, compressed, encrypted, multiplexed and modulated for transmission to a customer's home. As can be seen, this process leaves much to be desired as it requires a number of time-consuming steps that must be implemented with additional hardware, including magnetic tape recorders and players. Moreover, a large magnetic tape library must be maintained and indexed. Furthermore, the conversion from digital to analog and back to the digital domain may result in degradation of the signal quality.
Thus, it would be desirable to provide a system for allowing an auxiliary compressed digital signal such as a commercial message to be inserted into a compressed digital signal of a main program without requiring decompression of the data in the main program signal. Such a system should allow cable system headed operators to conveniently insert commercial messages into a main program which is received, for example, via a nationwide or international satellite distribution network. Additionally, the system should not degrade the quality of the main program. In particular, the system should avoid any discontinuity which results in a non-compliant data stream. The system should also preclude problems such as syntax violations, decoding errors, buffer overflow or underflow, timing recovery problems due to discontinuous system time stamps, audio/video synchronization problems, and video display artifacts. Furthermore, the system should be compatible with MPEG and similar digital data communication standards, in addition to being fully compatible with existing decoder technology. The present invention provides a system having the above and other advantages.